Grid control for static current converters



Aug. 16, 1949. u. LAMM GRID CQNTROL FOR STATIC CURRENT CONVERTERS Filed Dec. 23, 1944 I N V EN TOR. U027 57/77/77 A fink/75y Patented Aug. 16,1 949 GRID CONTROL FOR STATIC CURRENT CONVERTERS Uno Lamm, Ludvika, swedem assignor to Allmanna Svenska Elektriska Aktiebolaget, Yasteras, Sweden, a'Swedish corporation v V H a Application December 23, 1944, Serial No. 569,514

I In Sweden November'25, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires November'25, 1963 12 Claims. 1.,

This applicationis directed to improved means forthe 'c'ommutatiomof rectifiers in plural or polyphase systemsincluding those derived from a smgiep ase source in full-wave rectification f 7L ml. .VM'.

The commutation-procedure incurrent rectifiers;-invert'ers ,'-or frequency changers may, as regards'the obtainment of reliable operation, be divided into two principally difierent types. In one of these types; re'presentedfor instance by the spontaneous commutation of a rectifier or by the second stage of a forced commutation by means or a -condenser f the voltage available for fiectingthe' commutation rises during thewhole procedure, which means that a complete 1 commutation is always ensured: 'In the other type, presenting itself for-instance ininverters, operating at'acomparatively-high ratio between direct current andalternating-current voltage, or in the first stageof' a forced commutation bymeans of acondenser,the-voltage available for the commutation is lowered duringat least the main portionof the commutation procedure,-whence there is always a-riskof the commutation being '"not completed until too late. The voltage available forefiecti-ng the commutation is for instance; at a commutation between two anodes having -a common cathode, equal --to the voltage between the twdanodes. mediately before-the beginning of the commutation, be regarded as equal to the voltage' across the-relieving valve path (the valve path to be ignited), since the voltage across the'previously active valve pathrnay be neglected. During-the commutation procedure, the voltage is essentially consumed in the reactances of the commutation circuit;

The risk of non-completion of the commutation is particularly imminent at irregularities in the operation, for instance when the current is suddenly increased, whereby a" higher commutation voltage is necessitated, or when the shape of the voltage curve is occasionally altered so as to reduce the"commutationvoltage. An incomplete commutation orone which is not terminated until too lateis especially dangerous'in inverters where the consequences thereof are practically equivalent toa'short circuit. 1 7

' "According to the prese t invention, the commutationis' made independent of such incidents, which otherwise might prevent its termination in due time, by an arrangemenu which provides that the; control electrode potentialfifor example the: r d i e t al 9 the lievin t ai a instant of initiating-"thecommutation, is derived- Generally it may, at least imfrom-a voltage in the main circuit determining the commutation in such manner-as to follow the variations of the said voltage at the-said instant. For instance, the voltage across the relieving valve path before' the commutation may form a main component -of the said voltage determining the commutation. In many cases, it may be advisable to m'ake-the grid potential depend also on the time derivative of the last-mentioned voltage, for instance either so that-the derivative must be negative and the voltage at the same time inferiorto a certainvalue for efie'cting the ignition, orso'that the ignition depends on the ratiobetweenthe voltage and its time derivative,-said ratio being, for a rectilinearly'decreasing voltage, equal to the time-available for the commutation and for deionisation and blocking of the relieved valve'path; 7 a An electrode control according to the present invention offers in many cases other advantages than to make the commutation independent of incidental variations of the voltage curve. This i's-e speciallythe case in high tension current con verters, where comparatively expensive transmission channels are generally required between the more-or lessautomatic electrode control device for the normal operation, which usually operates substantially; at earth potential, and the control electrodes proper with their nearest accessories. In deriving the control electrode potentials according to the-present invention, the number of the said transmission channels can in most cases be considerabl-yreduced, for instance to one single channel for transmittinganappropriate comparison quantity, by the adjustment of which all the operational regulation is effected, as is'made clear by the following detailed description. In'the accompanying drawing;

while Figs. -2' -6' diagrammatically show different forms of the present invention. In Fig; 1, the curves I, IIQIII represent the voltages of forinstance the anodes of three valve paths operating in three-phase, said voltages being measured with respect to a neutral point, iffth cathodesar connected together, which is assumed in" the following lins'for the sake of simplicity; The heavy portions of the curves correspon'dto' the'time' intervals in which the respective valv'e paths are operative and the cathode thus has practically the same potential as the respective anode. When the commutation for instance from the path I to the path'II is to begin,

also'representsthe voltage between anode and Fig. 1 shows a voltage diagram of aninverter,

be initiated by unblocking the relieving path a certain time before the point of intersection. It is also well-known, that a certain voltage difference, i. e. an initial voltage across "the relieving valve path, is needed for the act of commutation, i. e. for overcoming the commutation reactances, and also that this voltage as well as the time of deionisation depend on the current, in view of which it may become necessary to make the in-- stant of initiating the commutation depend on the current.

As already said, it has long been well-known to;p ay. -attention to all these facts, and this is also very simple as long as the voltage curves retain their regular shape shown in Fig. 1 and .a fixed phase, position. It may, however, often happen that; the curve shape or the phase angle orrboth areincidentally altered in an unfavorable direction, sothat, witha fixed instant of initiating thecommutation or with one adjustable with the-loadgthe commutation cannot be completed, either because the, point of intersection is displaced backwards in time, or because the voltage is not sufficient to overcome the commutation reactances. Forpreventing this risk, the grid control is'so arranged, that the instantaneous value ofthe' grid potential of is derived from a voltage in the main. circuit determining the commutation. V

Fig. 2 shows a connection for this purpose which is particularly suitable for high-voltage current converters. of the-valvepaths of the converter, having a cathode I, a grid 2 and an anode 3, and the connection for grid control. The latter contains two auxiliary valves A, 5 of electronic valve type, i.-e.

vacuum tubes, soarranged thatonly one can be conducting at a time, and which alternatively makethe grid '2 positive (when the tube 5 is active) and negative (when the tube dis active). FOL-blocking the'tube 4 .and thus making the grid 2 ;positive, a transformer H] is employed, while impulses of the opposite character are transmittedby a transformer l I.

The impulses transmitted by the transformer l a re,-in this embodiment, produced by the following means. A high-ohmic resistance 13 is eonnectedbetween theanodeand grid of the main valve. A voltageris derivedfrom a portion of this resistance adjacent to the'gridand is impressed, in-oppositionto the voltage of transformer 56, on the gridof Vacuumtube H. The voltage of the transformer l may be derived from the main voltage over a phase :shiftingdevice in any manner-wellknown in the priorart. This transformervoltage-may have substantially the magnitude and-phase represented by-the dotted curve A in i- Figure'l. However, the. scale of curve A is magnified-vertically with respect tothe scale of the mainvoltage curves by a factor substantially eduallingthe-ratio-ofthewhole resistance l3 to the portion thereof-includedin thecontrol circuit. At the instantwhenthe commutation is to begin, the voltage of the-transformer obtains the same .value asthe voltage derived from the resistance I3,- and-theelectronic tube I! which is normally conducting becomes ,blocked, whereby 75 the relieving valve path The figure shows only one x 4 an extinguishing impulse is impressed on tube 4 by means of the transformer it. As previously stated, this means that the grid 2 of the main valve obtains a positive. voltage. and'this walve is therefore "ignited. The commutation therefore begins at the instant a, referring to Figure 1, which is defined by the condition that the voltage of the transformer; Hi obtains the same value as thevoltage derived from the resistance I3. The voltage on the grid of the vacuum tube I! then passes over to a negative value whereby the tube ll is blocked and thus interrupts the current through the primary winding of the transformer H3. Such an interruption will cause a steep voltage-impulsein the secondary winding, as is well known, and this impulse acts on the grid of the tube to block it. The anode of the tube 4 is then made positive by its connection to the plus terminal of an anode potential source through the resistance 6, and so is also the grid 2 of the main valvc 'which is :directly connected to .the anode of tube 4. The ,main valve is therefore ignited. The advantage of employing blocking-impulsesin the electronictubes :I1 and 4 for this-purpose is that these tubes, on account of the unavoidable inductance in the circuits, react :more rapidly for blocking impulses than for igniting impulses.

, The reason formaking the counter-voltage on the grid of the electronic tube, ll sine-shaped instead of constant is on one hand, that it should be possible, as an emergency step,,,to..ign,ite.the valve II (Fig. 1) at an earlier instant 17;,if the immediate preceding commutation has failed, in order to prevent a too heavy rush of current, and on theother hand, that an ignition at anybther instant shouldbe prevented, Ifthepreceding commutation has failed, the valve 'II.is .atthe instant b subjected to a voltage E, which represents the. difference. between the curves .III and II. At the same instant, this .voltage sinkslbelow the value represented by the curve A. The valve II is then ignited at the proper instant to afiord an ample time for commutation-of the increased current and .further, in -a two-way connection, to complete thiscornmutation before-the commute. tion of, the opposite -valve,path begins, for -avoid ing a simultaneous commutation of .tvvice the-current, andfinally also forpreventing the number of volt-seconds propelling this current from becoming excessive. Following the course lofsthe voltage curves, it=is foundthat acommutation is normally possible only at-the instant-a an'clin the event of a failure onlyat the;ins tant b- The derivation of the; counter-voltage .on the grid'of the electronic tube H in the manner now described has, however, the inconvenience, that it may be necessary to take special steps for preventing, in the case of a deformation or;sudden phase displacement of the, curve of the main openation voltage, such disturbances .from influencing also the curve A, wherebyzthe grid controlwould not fulfil its essentialpurpose. In orderto avoid this inconvenience, thesineshaped voltage-curive A'may be replacedby a. curve of such character,

asthe dash and dotted curveB. This curvemay, even if its superposed crests are: derived from-themain voltage, be made practically independentof any perturbations of this voltage .-At the same time there is required, however, for lthetrieason that the comparison voltage represented -.by .the curve, cannever become negative, a completion ,of

thecondition for initiating the commutation, for

instance thatthe time derivativeof the -voltage determiningthe commutation should be negative. A connection forzthispurpgse-is-shoym Fight,-

zwhioluonly Shows -:.the ame'ansatfor; obtaining. the

'primary ignition impulse, while the rests'of the grid control apparatus: mayibe: arranged according tooFigsZor-in any other manner." 1 1 -1 gInrFig, .3..'as; well. as :in Fig. .2,, the numeral l3 designates; a highohmic resistance; "which is supposed .to;be connected abetween'the anode of the mainvesseljand appoint-.witha potential near the cathodendnAt-the cathode end .ofthis'resiste ance and in series'thereWith, thereiis in Fig. 3 a small :inductance =|8. ..The:junctin' point hetweenaithe resistance and the inductance. is connected to a; point through a valve 19,:the point 2J1 being, Possibly through;a:vo1tage source,- con:- nected with a control .grid; corresponding for in stance to the; grid ;of theielectronic tube l l in R Th e valv e -l9 admits a currentsonly towards; the ;point 20. eThis point is also,;through a yresistanoeil; ;connected; to the lowerend of the inductance [8 (the end adjacentto the oathode) -,;;w hich is, regarded as of zero potential, and through; another resistance 22with a point have I iegative t potentialn Through a" valve 23, w ch admits .a. current nonlygtowards the point 29 said latter pointisconnected ,to a point .24 which fprrns the negative terminal of a .voltage sourcegivingthe voltage curve B in Fig. 1. This vgljiagesource consists ofa direct current voltage sour cefl in series with aisecondary winding of sjormer;;26 and :a valve 2 I admitting only alf; waye .of the alternating; current, while a. l1hfi1 aYQr es another secondary winding with a valve-acting in the opposite direction; In parallel to the sum ofthesestwo voltage sources, there; a voltagedependent resistance for ,in-

- theshapepf a; glow-discharge lamp 28,- uts; ofi any-total voltage lying. above a 1Bvel;:so as :to give; a "composite voltage thegtype represented by; the curve-B, of .c The-gdifierence between the voltage deccordin tothe curve 3 presents itself point 2.4; which; becomes negative whenqom autat o iis-to be initiated. E nn s ,p oint- 20 negative it is; however, also necessary t th toprend, of the inductance I8 becomes he: current. in ;the resistance I 3,- thus erivativeof the .voltage across the said r I w h oomesnegative If thoseztwo conditions are :lllfi edat ;;a tinietinocurrent-can t fersrs itheva ves-. 9 .1 ra d therefore the tential o thepointifi will be determined'by its ween the zero point and the negative e e is e celn 3 ,.;Th transformer-25 with its two secondary circuitscohtaihingvalves admitting the current in difierent;directionstgives ineach of these circuits a current containing only the half -waves of either Tn-the circuit containing the valve 2T and the esistance 32, a voltage is created which con- I' firis: only the half-waves of one sign; making the ap {the resistance32- positivewith'respect to th' ottom .To this voltage, the constant-one obt ned frojm the source 25 is added, "and this com osite voltage is impressed on the circuit for e d';by;the; glow discharge lamp '28 and the, resistance 33., The lamp has theycharacteristic prdperty that'any voltage above a certain limit impressed thereon strongly increases the current,

i ll h i -i tr fu rentw e t e Voltage drop; e re s nt? 3.3.,whiq ractica ly acts as out:

in A t any voltage above a certain limit. ,The

@83 1, h lyoltagecurve acros the glowdischarge; will therefgre besgonipqsed of: a. constant;

which occurs as soon as the time deriv basicavoltagesand trapezoidal half. waves: supers imposed thereon, thus of thenature of the :curve B in' Figurel. 1;. r1 p As stat'ed1above,:the ignition of the next valve; for. instanceathat presenting the voltage curve II infFig. :1, should take placeawhenithewoltage thereon; thus the .1 voltagev difference. between curves.II and I;fallsfbelowlacertain value repr'ee sented on a larger scale. by :the curve B, while at the: same time thev time derivative of. the said Voltage is negative. The first'one of these conditions is fulfilled-intherfollowing manner: ;A smalljfraotion 01fv the voltage across the resistance 13, iwhich is-supposed to be connected sub-'-. stantially in the-same manner as in'Fig. 2and thus represents'practically thevoltage across the ma-invalve, is obtained between the point 34 and the bottom of'the resistance. This fraction is small zas'. compared with the total voltage across the resistance [3. Practically astlong as the volt-'- age'between the point 34. and the lower end of the resistancel3-represented in Fig: 1 by the difference betweenithe'curves II- and I on ai'scale equaltoithatof the curve B-'is higherfthan that obtained from the lamp 28e-represented bythe cu'rve =B'.-a;current -Will thus .flow'from the :point 34 through the. lamp 28, resistance 35,? -valve'.'23,- resistance 2 I ,,.zero. conductor andvreactor l8'ba'ck toresistance I3. .This currentmakes thepoints 24 and :2 I positivev with respect to 'theszero con-v (111017012 As soon as the voltagebetween the point 34 and the lower end'of the resistance l3 drops below that across thelamp: 28;'on"the"contrary; a currentin the opposite direction isblocked by the valve 23, and unless a current flowsthrough the valve l.9,-*a's* hereinafter describechthepoint' 20Fbecomesnegative by its. connection through the-resistance 22' with'a lpoint having negative potential." 1 i t": The possibility. of a current flowing through the valve-I9 is. determine'd'nby the second'condition to berfulfilled; namelyjthat the time derivative of the voltage'across'the resistance I3 is negative. Aslong-as it is positive, the reactor l8 opposes th'e'ccurrenti-flowing "downwards through theresistance l3,-and the potential on the upper end of the reactor-is thus positive with'respect to the lower-or zero end; A current then flows through the'valve19, making the point 28 positive, irrespective. of theconditions to the left of saidpoint: As soon as the time'deriva'tive referred to becomes -negative,'the upperend of the reactor'l'a: also becomes negative and the valve l9 blocks the-current. If both valves I9 and'23 block,the point 20 becomes definitely negative and 'iinpresses a negative potential on the grid of tube which hasthe same function as'the tu-be IT in Figure :2 and-thus starts theignition of the main.- valve as $0011 asiits grid becomes negative. The direct-current voltage source 25, which" preferably may consist of arectifier'with a'device for keepingthevoltage constant and smoothening means .,and'the alternating current voltage source mayapreferably both 'be adjustable; For iacilitating'an adjustment from outside, especially in high voltage converters, in which it must beeffected by means of high tension insulators or other "comparatively complicated transmission means (channels) it may sometimes be preferred toremploy a connection according to -Fig. 4. This figure,"which fcorresponds to the left hand partof Fig.x.-3, has in common with the latter the voltagerdependentiresistance 28Jfo'r cutting off thezwavecrests of. the. voltage curve. The ends Qfaathis resistance arelconnected to two parallel vo1tage-sources, .one alternatingicurrent;source25 and onedirect current source 25, each across a valve 30 and 29 respectively, said valveslpermite ting atany instant .only the higher voltage to .act. Avoltage curveof the sametype. as in-Ei'g. '3 is thus obtained, but an adjustment of the .di-rectcurrent voltage only afiects the.=1ower=value of the. curve B, while the higher one is determined by. the alternating current voltage only. The voltage-dependent. resistance 28 may here. be omitted if the p-rimaryvoltage or the transformer 25 has a flattened curve shape.

It may alsobe possible to add, according-to Fig.- 5, a fiattenedalternating current from a transiormer 28, which maybe corrected bymeans of. a voltage-dependent resistance 31, .simp yi to adirect current voltage from a source 2,55and.to impress the sum on :the. pointild in Fig. 3.-

Itrmay be sufiicientto employ azconstant com.-. parison voltage in normal operation and 'to'raise this yoltage .only on the .occasion .of a commutation failure. Such a raising may. be effected practically.instantaneously ebythe aid of a valve or..a voltage-dependent resistance influencingthe comparison voltage, said valve or resistancebeing influenced by a consecutive phenomenon of the commutation failure, for instance anabnorm-al voltage. or current. In two-way converters, .the circumstance. that opposite anodes carry current simultaneously maybe employedfor this purpose, as this is not :normallythe case. In this. arrange.- ment, no transmission from parts of:considerably di-fierent .ipotential is necessary.

In some cases, itmay be advisable .to initiate the. commutation at: an instantiwhen a certain proportion prevails between the voltage determine ing the commutation .and its (negative) time derivative. When the iormoi. the voltage curve does not too much differ from a straightline,-, the said proportion forms a .measure of the time available for the commutation; Aconnectionior this purpose isillustrate'd in Fig. 6;.which shows amain valve, aresistance i3. .connectedrin series with an inductance I8 :between the anode 3:and cathode of saidvalve, and .an electronic tube H; the cathode of which is connected to the cathode of the main valve, while. its grid is connected to anoint. on the resistance .13. Atuthe instant; when the voltage across :the portion of :the resistiance lying. between the tube grid and the react-- ance .becomes equal .to the voltage. .across :the in..-v

ductance, determined by the decreaseof the current, the point connected to the grid .ofthe tube; l-llobtains zero potential, whereby. the .tubemay be blocked. If it .is desired tovaryithe iproportion between the voltage anditstimederivative at which this occurs, the bottom portion 10f the-resistance [3 may be in the. form of avoltage dependent resistance which is varied by van impressed cross-voltage, either. .on a prismatic resistance, bar or in abridge .con-nectionubetweenfour ,resistancesconnectedin tetragon.

If, inanyof the connections according to Figs.- 3-6, it is desired tofemploy the bottomiportion ofthe resistance l3 for derivingan extinguishing-- impulse .ior the main valve, as has-been indicatedin Fig. '3, the inductance :18 may :be placed above insteadof below the said portion. The .modifica tions .of/the connection involved'herebysare obvious to anyone skilled in the art. 'Insteadiof reproducing the time derivative-of-zthe voltage by means of an inductance traversed bythecurrent, the voltage itself maybe impressed von a condenser in series with ,a resistance and the voltage-- derived from this resistance. The iorm: shown using an inductance-is, however, in most. cases simpler.-

The: invention is-ofi course applicable on any kind of grid control. where the commutation can be initiated bya'yoltage passing through acertain value. Theexpression -grid control then also includes. the control, in most cases fully equivalent. therewith, by. means of rapidly acting ignition anodes .(ignitors) determining the mstantaof igniting the valve path.

:The-controldescribed which depends on the instantaneous .values :of the voltage determining the. commutationand/or its time derivative-may he-combmed in different manners with other control means ior. instance the usual, generally timedependent one. The control means which immediately-.influencesi-the grids, which may receive impulses from difierent devices-then automatically-actinsuch manner that on each commutatingoccasion-eny impulse arriving afterthe starting of the commutation becomes inactive. For instance, in an inverter or frequency changer, the gridcontrol ofuthe normal operation may be efiectedlina manner known before, while-the control .accordingto the present-invention is-kept inreserve seas-to enter intoiunction if the normal (control insome case-might not be able to-efiect a commutation. In this case, the adjustable quantity, forinstance a comparison voltage, whichudetermines the instant of commutation, oi'course-may pay respe'ct to-all factors influencing the duration of the commutation, in the first line .oficourse to the current.

.I claim as myinvention:

1. In apolyphase'current converter including at leastione ignition electrode-controlled rectifier pathfor each =phase, .a network-having input terminals and output terminals, said input terminals being connected-Ito -asource of varying potential dependent on t he instantaneous difference of the voltages in the relieved and the relieving rectifier paths andto a comparison voltage, said network including means to supply to its output terminals apotential proportional to the difference-between such comparison voltage and such instantaneous difference, andsaid output terminals being connected in the ignition circuit of the relieving rectifi'er path;-whereby the firing time of said relieving rectifier path is determined by the instantaneousvariations of the voltage difference between relieving I and relieved rectifier paths.

2. in a poly-phase current rectification system comprising ait least two electrode-controlled rec'- tifier paths eachhavingacathode, a control electrode, and an anode, and arranged to conduct in sequential 0111612 2) network 'having input terminals 1 connected -inthe anode-cathode path of the-rel-ieved-rectifier pathand to a comparison voltage; said networkincluding means to supply to its output terminals a potential proportional to the =di-fferenc e b'etween such comparison voltage and the instantaneous voltage of thelrelieved rectifier path, and havingthe output terminals connected in the control electrode circuit of the relieving-rectifier path, whereby the control electrodepotential oi the relieving-rectifier path is aifunotion-of; ithe instantaneous variations of the anode-cathode potentials ofthe said relieved rectifier path. V l i 3.1 In a poly-phase current rectification system, a; plurality :of electrode-controlled rectifier paths, each having atgleast-a-cathode, a control elecr trode, and'fl'ananode; a'network-havingin put and output terminals-said input terminals being con nected to source'sof'a first potential'and a second potential, said first potential being proportional to the difference-and said second potential being proportional to the time rate of change respectively of the instantaneous voltages applied to the relieving and relieved rectifier paths; said net- -work including means to furnish to its output terminals a potential proportional to the algebraic sum ofjthe first and second potentials said output terminals being connected in the control electrode circuit oi the relieving rectifier path to render the same conducting atv a predetermined algebraic sum of the said first and second'potentials, whereby the control electrode potential of Z the said relieving rectifier path variescas the ratio between themain voltage difference between the relieving ages applied to the relieving and relieved rectifier paths for varying said voltage-dependent resistance portion, said network having output connections applied to said control electrode, whereby the control electrode potential of the said relieving rectifier path varies as the ratio between the main voltage difference between the relieving and relieved rectifier paths and the time derivative of said difference.

5. In a polyphase current rectification system including electrode-controlled rectifiers each having at least a cathode, control electrode, and anode, and arranged to conduct in sequence whereby at intervals one of said rectifiers is being relieved of the load and one of said rectifiers is assuming said load, a network having input terminals connected to a resistance responsive to the voltage difference between the relieving and relieved rectifier paths, having other input terminals connected to means for supplying a constant potential and still other input terminals connected to the ends of a reactor supplied by a current proportional to the aforesaid voltage difference, said network having output terminals connected to a valve determining the firing control electrode potential, said network being adapted to supply a potential to the output terminals when the magnitude of voltage difierence between the relieving and relieved rectifier paths passes through a selected value, and the potential across said reactance is that corresponding to a decreasing current. V

6. In a polyphase electrical system, a polyphase source of potentials, and a plurality of electrodecontrolled rectifier paths, said system having at least one rectifier in each phase, means for controlling the ignition point of said rectifiers whereby they conduct in sequence and whereby at a particular instant one rectifier is relieving a relieved rectifier of its load, a network having input terminals connected to a source of a first potential which is a portion of the potential between the relieving and relieved rectifier paths, and in opposition thereto to a source of a second potential which is of a comparison voltage, and having means to supply to its output terminals a potential proportional to the difference between said first and second potentials said network opposed voltages.

. 1'0 having output terminals applied to the control electrode of said relieving rectifier path whereby said relieving rectifier path becomes conductive at a preselected potential difference between said first and said secondopposed potentials. I

'7. In-a polyphase current rectification system wherein a plurality of electrode-controlled rectifier pathsv are arranged to conductin sequence and at a particular instant of commutation one rectifier, path may be termed the relieved rectifier path and a secondrectifier path next in sequential order may be termed the relieving rectifier path; a network having applied thereto in oppo sitiona portion of the voltage difierence between the relieving and relieved rectifier paths, and ,a voltage similar infrequency and wave shape to one of the polyphase voltages, and having means to supply to itsoutput terminals a voltage proportional to the voltage difierence between said opposed. voltages, the output terminals: of. said network being connectedtothe control electrode of said relieving rectifier path whereby said relieving, rectifier path is rendered conducting at a predetermined voltage difference between the said.-

8. In a polyphase grid-controlled rectification system including at least a relieving electrodecontrolled rectifier path and a relieved electrode grid-controlled rectifier path; a network having a first and a second input, and a source of alternating voltage derived from one phase of said polyphase system connected to said first input and a second source of voltage having a value proportional to the voltage difierence between the relieving and relieved rectifier paths connected to the second input and having means to supply to its output a voltage which is a function of the voltages applied to said first and second inputs; the output of said network being connected in the control electrode circuit of said relieving tube; whereby the igniting voltage applied to said control electrode is a function of the voltages applied to said first and said second inputs.

9. In a polyphase static current converter including at least a first and a second electrodecontrolled rectifier path arranged to conduct in sequence whereby at any instant of commutation there is a relieving rectifier path and a relieved rectifier path; a network connected to a source of constant voltage and to a source of periodical voltage in addition thereto, and in opposition to the sum thereof to a source of voltage proportional to the voltage diiference between the relieving and relieved rectifier paths and having means to supply to its output a voltage which is proportional to the algebraic sum of the voltages impressed on the network, the output of said network being connected in the controlled-electrode circuit of the relieving rectifier path whereby the same will conduct when the algebraic sum of the voltages impressed on said network is equal to the igniting electrode potential of the tube.

10. In a polyphase static current converter having a plurality of electrode-controlled rectifier paths and at a particular instant a relieving rectifier path and a relieved rectifier path; a first and second network; said first network containing therein a source of constant voltage and being connected to a source of alternating voltage derived from one phase of said polyphase system, and being arranged to add the two voltages together whereby the output of said network contains the sum of said voltages; said second network being connected to a source of voltage proportional to the voltage difference between the,

relieving and relieved rectifier paths and the output voltage of said first network and being arranged to supply to its output a voltage which is a; function of the instantaneous magnitudes of the voltages applied to said networks; the output of the said second network being connected in the control electrode circuit of the relieving rectifier path.

' r 11. In a polyphase static current converter having at least one electrode-controlled gaseous or i vapor rectifier path for each phase whereby at any instant of commutation there is a relieving rectifier path and a relieved rectifier path; a resistance having a tap thereonand connected in series with an inductance connected in an anodecathode path of the relieving rectifierpath, means for comparing the voltage derivedfrom said resistance tap and the time rate of change voltage derived from the junction of said resistance and the end of the inductance and means for applying the resulting voltage to the control electrode ofthe relieving rectifier path.

' 152. In a polyphase static current converter having at least one electrode-controlled gaseous or vapor rectifier path in each phase and having at 25 an instant of commutation a relieving, rectifier pathand a relieved rectifier path; an electrodecontrol voltage deriving network connected to ,a first source of potential proportional to the voltage difference between said relieving and said relieved rectifier paths, means for impressing thereon a second potential which is the time ,derivative of said voltage difierence and having an output terminal connected ,to said ,network through two paths, a rectifier in each path having impressed thereon said first and second potentials respectively andbeing sonpolarized that a positive electrode-control potentialwat thesoutput' terminal is determined by the simultaneous r. fulfilling ofone potential condition for, each path.

,UNO LAMlVI.

I file of this patent:

UNITED STATES PATENTS Name 7 Date -Lamm June 7, 1938 Number 

